Coccidiosis is a disease of poultry caused by intracellular protozoan parasites of the genus Eimeria. The disease is endemic in large, intensive poultry breeding establishments. The estimated cost of control of the disease through chemotherapy exceeds $100 million each year in the United States of America alone. The development of resistance to the known anti-coccidial drugs necessitates a continuing development of new agents, at a time when drug development is becoming increasingly expensive and consumer acceptance of drug residues in food animals is diminishing.
Protective immunity to natural coccidiosis infection has been well documented. Controlled, daily administration of small numbers of viable oocysts for several weeks has been shown to result in complete immunity to a challenge infection of a normally virulent dose [Rose et al., Parasitology 73:25 (1976); Rose et al., Parasitology 88:199 (1984)]. The demonstration of acquired resistance to infection suggests the possibility of constructing a vaccine to induce immunity in young chickens, circumventing the need for chemical coccidiostats. In fact, such a concept has been tested in the Coccivac.TM. formulation of Sterwin Laboratories, Opelika, Ala.
With a view to producing a coccidiosis vaccine, Murray et al., European Patent Application, Publication No. 167,443, prepared extracts from sporozoites or sporulated oocysts of Eimeria tenella which contain at least 15 polypeptides, many of which were associated with the surface of the sporozoite. Injection of these extracts into chickens reduced cecal lesions following oral inoculation with virulent E. tenella sporulated oocysts.
More recently, Schenkel et al., U.S. Pat. No. 4,650,676, disclosed the production of monoclonal antibodies against E. tenella merozoites. Using these antibodies, Schenkel et al. identified a number of antigens against which the antibodies were directed. By pre-incubating E. tenella sporozoites with these antibodies and then introducing the treated sporozoites into the ceca of chickens, Schenkel et al. were able to show some reduction in cecal lesion scores, compared to untreated sporozoite controls.
Using recombinant DNA methodology, Newman et al. (European Patent Application, Publication No. 164 176) have cloned a gene from the sporozoite stage coding for a 25,000 dalton antigen from Eimeria tenella. Sera from chickens immunized by repeated immunization with killed E. tenella sporozoites immunoprecipitated this antigen from iodinated sporocyst and sporozoite membrane preparations. More recently, Jenkins [Nucleic Acids Res. 16:9863 (1988)] has described a cDNA encoding a part of a 250,000 dalton merozoite surface protein from Eimeria acervulina. The expression product of this cDNA was recognized by antiserum against the organism.
Advances in recombinant DNA technology have made another approach available, i.e. a subunit vaccine. Examples of such subunit vaccines are described e.g. in European Patent Application, Publication Nos. 324 648, 337 589 and 344 808.